The present invention relates to a symmetrical bridge circuit useful in a mass flow air sensor to measure the mass of airflow into an internal combustion engine.
The basic measurements in fuel injection systems are the measurements for determining the amount of fuel to be supplied to the engine. Air/fuel ratios are satisfied by measuring the amount of air intake to the engine and then supplying the proper amount of fuel under control of scheduling tables located in the electronic control unit (ECU).
The most common method to determine the amount of fuel to be injected is to measure the manifold pressure and engine speed and from these measurements determine the amount of fuel. Hot wire anemometers and swirl meters are examples of devices for measuring the amount of air flowing into the engine and with this measurement, the amount of fuel is calculated.
In many devices, a bridge circuit is used in the measuring circuitry. The bridge consists of a heated leg responsive to air flow and a unheated leg responsive to air temperature. The unheated leg is used to compensate the anemometer for changes in air temperature. The unheated leg must not be heated by the circuit. In some, a low resistance is used in the heated leg and a high resistance element is used in the unheated leg. In these situations, both elements are operated at the same voltage requiring the matching of the temperature coefficients of the resistance material with the different values of resistance. It is desirable to use the same type element in both legs.
A modification of the above mentioned bridge circuit is to divide the voltage to the leg containing the unheated resistance element and then amplify or multiply the voltage output to cancel the division. This generally requires the use of capacitor compensation to assure stability of the gain stage of the amplifier which in turn slows down the circuit response.
Still other solutions have placed the unheated resistance element in the feedback circuit of an amplifier in the bridge circuit. This requires a number of interactive function adjustments and a regulated voltage reference.
In my invention illustrated in U.S. Pat. No. 4,637,251, I have proposed a current bridge circuit which operates by balancing the current flow between a heated and cold leg of a resistance bridge which operates over a wide range of air flow to maintain a heated resistor at a given increased temperature differential above a colder resistor. In this case both the heated and cold resistor are temperature sensitive resistors. The operative circuit as illustrated in FIG. 3 of my patent requires for its operation the precise balancing of a large number of resistors. As such, in a production environment this circuit may prove to be relatively difficult to adjust the various resistors to compensate for differences in the temperature coefficient of resistance of the hot and cold resistors and also to modify these resistors to maintain a precise and desired temperature differential of the heated resistor.
In contrast, the present invention provides for a relatively simple and straight forward current bridge arrangement requiring very few, if any, adjustments to a select few resistors to obtain the required temperature differential and compensation for variations in the temperature coefficient of resistance of the heated and cold resistors.
It is an object of the present invention to sense airflow by balancing the current flow in a current bridge arrangement.
Accordingly the invention comprises a circuit comprising:
power driver means, adapted to be connected to a voltage source for supplying current in response to a control signal; PA1 means for maintaining a desired temperature offset between a first temperature variable resistor and a second temperature variable resistor comprising: PA1 a current bridge (14) adapted to be positioned in an air-flow comprising a sensing leg (16) including the first temperature variable resistor (R.sub.h) and a first offset resistor (R.sub.20) and a compensation leg (18) including the second temperature variable resistor (R.sub.c) and a second offset resistor (R.sub.22), PA1 current stabilizing means connected to the sensing leg (16) and compensation leg (18) for causing the current flow in such legs to be at a specified ratio, including a first amplifier (A.sub.1) having its positive input connected to a first resistor (R.sub.19) which is connected in series with the sensing leg and a second resistor (R.sub.21), the output of the first amplifier is connected to a first transistor (T.sub.s) which completes a series connection between the compensation leg and the second resistor, these first and second resistors are chosen to generate such specified ratio; PA1 a first voltage divider is connected across the sensing leg, comprising third and fourth resistors having approximately the same ratio as the first and second resistors; PA1 a second amplifier having a positive input connected to a junction between the third and fourth resistors and its negative input connected to sense the voltage across the compensation leg, and an output of the second amplifier communicated to and used to regulate the power driver means.
Many other objects and purposes of the invention will be clear from the following detailed description of the drawings.